1. Field of the Invention
The present invention relates generally to heat pumps, and more particularly to a heat pump system which diminishes the irreversible energy losses that occur during heat exchange.
2. Description of the Prior Art
A heat pump system which produces a high-temperature source fluid, such as hot water, by making use of a low-temperature source fluid, such as industrial waste water, has heretofore been known. In particular, a heat pump system of the compression type in which the compressor is driven by means of an electric motor or a heat engine is now in wide use because of the availability of heat energy that reaches even several times the power input.
However, when the low-temperature source fluid or the high-temperature source fluid is a single-phase fluid such as water without phase change, performance of the system has been limited. Explaining the situation based on FIG. 1 which describes temperature variations during heat exchange between source fluid and a single-component working medium for a prior art system, the abscissa shows the amount of heat exchanged and the ordinate shows the temperature. In the figure, the segment T.sub.e represents the temperature during the evaporation process of the working medium, the segment T.sub.c the temperature in the condensation process of the working medium, the segment T.sub.A the temperature variation of the high-temperature source fluid, and the segment T.sub.B the temperature variation of the low-temperature source fluid, respectively. Like in the above, a single-component working medium possesses a fixed boiling point so that its temperature remains unchanged during its process of evaporation or condensation. In contrast, the temperature of a single-phase source fluid varies along the direction of its flow during the process of heat exchange. Because of this, the hatched portions of FIG. 1 remain as the irreversible energy losses during the heat exchange, giving a limitation on the effort for improving the performance of the system.
To cope with this situation, use of a non-azeotropic mixture as the working medium has been proposed. With a non-azeotropic mixture which is obtained by mixing single-component media at a fixed ratio, it becomes possible to vary the temperature, both in the processes of evaporation and condensation, in the manner as shown by the segments T.sub.d and T.sub.f, by making an advantageous use of the difference between the boiling points of the two media. Then, it becomes possible to reduce the temperature differences between the working medium and the source fluids during heat exchange, suppressing the irreversible energy losses.
However, the use of such a non-azeotropic mixture has not been put into a wide-spread practical use for reasons such as the technical difficulty in restoring the mixture composition to the initially set composition when the mixture leaks from the system.
In addition, as a heat pump system of other kind, there has been known a cascaded heat pump system which is obtained by coupling a low-temperature cycle to a high-temperature cycle with a cascading heat exchange. The cascaded heat pump system permits the range of temperature rise to be set at a large value. Thus, for example, it is possible to generate hot water of over 150.degree. C., or the like, by the use of 30.degree. C. to 60.degree. C. industrial waste water for the low-temperature source fluid. However, as in the heat pump system described above, the cascaded heat pump system suffers from a certain limitation in the effort to improve the performance in the case when a single-phase fluid like water without phase change is used for the low or high-temperature source fluid.
This may be explained based on FIG. 2. In this figure, the temperature variations during the heat exchange between the source fluids and the working media are shown for the case when single-component working media are used for both the high-temperature cycle and the low-temperature cycle, where the abscissa is the amount of heat exchanged and the ordinate is the temperature. The segment T.sub.e represents the temperature of the working medium during the evaporation process in the low-temperature cycle; segment T.sub.c represents the temperature during the condensation process in the high-temperature cycle; segment T.sub.B represents the temperature variation of the low temperature source fluid; segment T.sub.A represents the temperature variation of the high-temperature source fluid; segment T.sub.p represents the temperature of the working medium on the low-temperature cycle side in the cascading heat exchanger, and segment T.sub.q represents the temperature of the working medium on the high-temperature cycle side in the cascading heat exchanger. As seen in the figure, in contrast to the constancy of temperature during the process of evaporation or condensation of a single-component working medium which possesses a fixed boiling point, the temperatures of single-phase source fluids during the heat exchange vary along the flow of the fluid. Because of this, the hatched portions of FIG. 2 become irreversible energy losses during the heat exchange, giving a limitation on the effort for improving the performance of the system.
It has also been proposed to utilize a non-azeotropic mixture as the working medium in a cascaded heat pump system. A non-azeotropic mixture obtained by mixing single-component media at a fixed ratio is aimed at introducing temperature variations in either the evaporation process or the condensation process by means of the difference in the boiling points of the two media. Therefore, by utilizing a non-azeotropic mixture as the working medium and by arranging for it to flow countercurrent-wise with respect to the source fluid to carry out heat exchange, the temperature difference during heat exchange between the working medium and the source fluid can be made small as represented by the segment T.sub.d with respect to the segment T.sub.B, making it possible to reduce the irreversible energy loss.
However, refrigerants such as R11 or R114, that can be chosen as components of a non-azeotropic mixture may only be suitable up to about 120.degree. C. of high-temperature output due to reasons of thermal stability and the like. Because of this, use of a non-azeotropic mixture in the cascaded heat pump system is limited to the low-temperature cycle alone, necessitating the use of a single-component medium for the high-temperature side.
Moreover, in a cascaded heat pump system with high-temperature output, water vapor is sometimes generated in the high-temperature cycle condenser. When water vapor is generated in this way, the temperature of the high-temperature source fluid, instead of changing in the direction of the fluid flow, behaves as shown by the segment T.sub.R due to the evaporation that accompanies the vapor generation in the condenser. Owing to this, even when the temperature of the working medium does not change in the condensation process, the temperature difference between the working medium and the high-temperature source fluid will not widen, and hence, the irreversible energy loss during heat exchange will not increase. Accordingly, there will be found no inevitability in such a case for using a non-azeotropic mixture on the high-temperature side.
Furthermore, when a non-azeotropic mixture is used for the low-temperature cycle and a single-component medium is used for the high-temperature cycle in a cascading heat exchanger, the single-component medium stays in its evaporation process at a constant temperature as represented by the segment T.sub.q, while the non-azeotropic mixture during its condensation process decreases its temperature as shown by the segment T.sub.f. For this reason, the temperature difference between the non-azeotropic mixture and the single-component medium, during the heat exchange process in the cascading heat exchanger, widens, thereby increasing the irreversible energy loss in the process and thereby resulting in a problem that the special features of the non-azeotropic mixture fail to be fully utilized.